1. Field of the Invention
The present invention relates to a resin-made throttle body which provides an air passage and a method of manufacturing the same.
2. Description of Related Art
Recently, a throttle body made of resinous material has been proposed to meet the demand for a reduction in the weight and cost of the throttle valve control unit. However, because the shape of the outer periphery of the throttle body enclosing the throttle valve is simply cylindrical, except for a throttle shaft holder, the molecular orientation of the resinous material may be formed in the direction that the resinous material is fed to a molding die. If glass fiber or carbon fiber is mixed to the resinous material to reinforce the throttle body, the fiber will also be oriented in the feeding direction.
If the resinous material is oriented in the throttle body, thermal expansion and contraction of the throttle body varies with direction. The orientation difference makes it difficult to provide a suitable clearance around the throttle valve. For example, if the throttle body has a round throttle valve therein, it is necessary to have a cylindrical inner periphery. However, a resin-made throttle body having an orientation difference as described above may deform, making it difficult to accurately control the air-flow rate, particularly, a small air-flow rate. Because the throttle valve also expands or contracts, the throttle body is required to expand or contract to the same degree to maintain the clearance. Thus, this orientation difference is an obstacle to the accurate control of the flow rate.
If the inner periphery of the throttle body expands or contracts differently in different directions, a portion of the throttle body may come into contact with the throttle valve, thereby hampering smooth rotation of the throttle valve.
JP-B2-7-51902 discloses a throttle body in which a metal ring is insert-molded into the inner periphery of the throttle body around the throttle valve. This eliminates the problems caused by the orientation difference of the throttle body, thereby providing an accurate clearance between the outer periphery of the throttle valve and the inner periphery of the throttle body.
However, inserting a metal ring into the throttle body hinders the reduction of the weight and cost of the throttle body.